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Tnx. Yes, I didn't take inertia into account as different weights and different weight distributions would corrupt the results. So in theory you would have to take into account and subtract the force needed to spin them in the first place. It starts getting ugly. All of these thoughts started because I'm in the process of ordering a new set of traditional wheels where I can decide the spoke arrangement and rim profile heights. I will probably go for a smaller rim at the front with radial spokes and taller profile at the back with x3 pattern. Classic. Cheers
Bugno
No. The spin and run-down test tends to be dominated by moment of inertia. After you've equalised for that, by ballasting all the wheels to have the same I, and taken bearing friction out of the equation by mounting all the wheels on a reference axle, you end up measuring mainly spoke drag, so spoke section and spoke count tend to be favoured over rim shape.
If you measure drag of an isolated wheel in the wind tunnel, all you're measuring is the drag of an isolated wheel. Not very useful, since the wheels interact so much with the rest of the bike. You can generate a ranking of wheels that way, and people do, but it's a crude measure. Usually people measure power needed to traverse (and maybe rotate, although few bother with this even though it turns out to be quite significant) at a given speed, giving results ranging from <20W to >30W per wheel at 50km/h. If there's a huge difference on the isolated test stand, it's pretty likely that the rank order will not change when mounted on a bike, but the real fight is over single Watts, and for a group of wheels which are all within ±1W on a test fixture, the rank order is likely to be different on each complete bike to which they are fitted.